This invention relates to an image sharpness detecting system for automatic focus adjustment and apparatus utilizing the same suitable for use in optical instruments such as cameras.
The system for detecting an optimum degree of sharpness of an image by utilizing the non-linearity of photoelectric effect prominent in photoconductive substance such as cadmium sulfide and cadmium selenide is already known, for example, in Japanese Patent Publications Nos. 39-29120, 41-14500 and 44-9501. The aforesaid non-linearity of photoelectric effect of photoconductive substance is a phenomenon that as the degree of sharpness of an image formed on the surface of the photoconductive substance is increased, the electrical response and particularly the resistance of the photoconductive surface is increased or decreased. This phenomenon is based on the fact that the amount of incident light per an elementary area of the photoconductive surface varies with variation in sharpness of the image, and the distribution of such incident light over all the elementary areas remarkably differs when the degree of image sharpness reaches a maximum value. In other words, the difference between light intensities of two adjacent elementary areas is largest when the sharpest image is formed, causing production of a large difference between the resistances of the adjacent elementary areas of the photoconductive surface. Most of the objects which may be usually encountered when photographing, however, have very irregular brightness distributions, and further the luminance ratio between two adjacent elementary areas or the relative contrast in the image is not always sufficiently high. Consequently, with the image-receiving surface of a photoconductive cell of a geometry which has been used in the prior art, it is difficult to accurately detect an optimum degree of sharpness of the image formed thereon owing to insufficient action of the photoconductive effect.
The structure of a real image projected onto the photoconductive surface from a usual object is complicated as including various luminance patterns so that the photoconductive surface of conventional geometry cannot sense the sharpness of all of these patterns but the sharpness of those portions which assume certain angular positions with respect to the electrodes of the photoconductive cell. For example, an optimum degree of sharpness of that portion of the projected image which is perpendicular or parallel to the electrodes of a photoconductive cell cannot be detected at all. On this account, it is necessary to impart a particular geometry to the photoconductive cell such that it can sense the sharpness of an image which may assume any angular position relative to the electrodes.
Another drawback of the prior art image sharpness detecting system is that when the total light intensity of the image is relatively low, an optimum degree of sharpness of an image cannot be detected accurately and reliably. This is because the electrical response of the photoconductive cell is small at the low light levels, and because the output of the photoconductive cell varies slowly at the point of sharpest focus of the image.